Defect inspection of semiconductor wafers and masks for IC manufacturing is an accepted production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective dies for repair, or improve wafer processing parameters. The systems used for inspection were typically optical system in nature and such systems were limited in resolution due to the wavelength of measuring light. Electron beam systems can have much higher resolution than optical systems because the wavelength of the electron can be in the angstrom regime. However, the electron beam systems are limited in speed at which they can inspect wafers. Depending on the scanning frequency, the present systems with throughput varying from 20 to 40 square cm per hour have been achieved. Thus to inspect an entire 300 mm diameter silicon wafer, at least 18 hours will be required. In order to achieve a throughput of several wafers per hour, (which is suitable for in-line use in a semiconductor fabrication facility), the idea of multi-axis magnetic lens system for charged particle beam was introduced.
The first multi electron beam patent was granted to Maekawa et al. in 1973 for throughput improvement exposure system. The apparatus consists of one common exciting coil and two magnetic conductor plates with a plurality of through holes for the corresponding particle beam passing, which was proposed in U.S. Pat. No. 3,715,580 and was illustrated in FIG. 1A. Between a pair of holes in the upper magnetic conductor plate 40 and the corresponding hole in the lower magnetic conductor plate 41 such as hole 4 and hole 5, a sub-lens such as 10 is formed. The two magnetic conductor plates 40 and 41 are the pole pieces of these sub-lenses. For this type lens, the magnetic field of the sub-lens is fundamentally different from each other in magnetic field distribution and magnetic field strength (FIG. 1B).
Comparing with the conventional single-axis magnetic lens, the field distribution of the sub-lens degenerates from axial symmetry to rotation symmetry and/or n-fold symmetry (FIG. 1B, FIG. 1C). As a result, besides the axisymmetric field, the non-axisymmetric transverse field components called as high order harmonics such as dipole field (11, 12, 31, 32 in FIG. 1B) and quadrupole field 42 (FIG. 1C) appear. The dipole field deflects the particle beams, makes the beam land on the imaging plane with an additional transverse shift, an additional tilt angle and additional aberrations. The quadrupole field adds astigmatism to the beam. The appearance of the harmonics requires the optics includes at least one additional element generating the same type field to compensate its influence.
The magnetic flux passing through the center sub-lens 20 and peripheral sub-lens 10, are not same because they are located in the area of the two magnetic conductor plates 40 and 41 with different magnetic flux. This difference makes the axisymmetric field called as round lens field of the center sub-lens 20 a little different from that of the peripheral sub-lens 10, 30. As a result, the center beam 2 passing through the center sub-lens 20 and the peripheral beams 1 and 3 passing through the peripheral sub-lenses 10 and 30 respectively are not focused at the same imaging plane (FIG. 1A).
Many scientists who followed Maekawa's foot steps have tried many methods to eliminate the influence of the harmonics and the round lens field difference among all the sub-lenses.
U.S. Pat. No. 6,750,455 of Lo et al. reduces the dipole field itself by using plurality of dummy holes to improve the local structure symmetry of the sub-lens the beam will really pass.
U.S. Pat. No. 6,777,694 of Haraguchi compensates the dipole field influence by inserting a deflector group in each sub-lens hole.
U.S. Pat. No. 6,703,624 of Haraguchi et al. nulls the round lens field difference among all the sub-lenses by changing the pole piece diameter or the pole piece gap size of the sub-lens to control the magnetic flux leakage in the individual sub-lens.
U.S. Pat. No. 6,703,624 of Haraguchi et al. and U.S. Pat. No. 7,253,417 of Frosien et al. compensate the round lens field difference by inserting an auxiliary round coil or an electrostatic lens in each sub-lens.
These previous methods were either making the magnetic conductor plate become larger and the multi-axis magnetic lens system bulky or making it complicated. The present invention will provide a better solution that eliminates high order harmonics along the charged particle beam path. Thereafter it provides a high throughput inspection tool for semiconductor yield enhancement.